Horizontal fiber bands in primate area 17 occur in layers I, IVB (also termed IIIC) and V, and are composed in large measure of intrinsic connections. Recent studies in our laboratory have revealed periodically organized intrinsic connections within striate cortex of tree shrews, squirrel monkeys, and macaque. After intracortical injections of HRP, these periodicities are prominent in the supragranular layers. (I-III). In monkey, further periodic connections are found within layers IVB and VI. These intrinsic anatomical connections have not been previously described. Their regular geometric pattern is strongly reminiscent of physiological and metabolic patterns of cortical activity; and these anatomical connections thus raise interesting implications for how functional properties (such as orientation specificity, binocularity, or complex receptive field attributes) may be elaborated intracortically. Periodic patterns also occur in intrinsic connections of prestriate cortex (areas V2 and V4), and in several extrinsic connections of both striate and prestriate cortex. Systematic analysis of these interrelationships is relevant to basic questions concerning the functional as well as structural organization of visual cortex. For instance, the occurrence of multiple, laminar-specific, intrinsic lattice-like connections questions the concept of radial columns or modules as the sole building blocks of neocortex and suggests that horizontal intralaminar connections are also important. The proposed investigations will concentrate on 1) providing a detailed light microscopic description of intrinsic periodic connections in striate cortex and in several prestriate areas (by means of HRP or 3H-amino acid injections), 2) analysing the relationship between selected pairs of intrinsic and extrinsic periodic connections within the supragranular layers, both in striate and prestriate cortex (by means of two anatomical tracers used in combination), and 3) initiating ultrastructural studies, with emphasis on the post-synaptic contacts made by HRP-filled neurons within the supragranular connectional lattice in area 17. It is hoped that these data will increase our understanding of cortical processes going on beyond layer IV, including subsequent relays to prestriate cortex, and provide some clues as to mechanisms of higher visual processing.